In UMTS (Universal Mobile Telecommunications Service) data processing, data pertaining to different propagation paths reach a mobile station at different instants of time. Moreover, said data may reach the mobile station from the same or different transmitting sources, i.e. base stations. The information transmitted in each path in the air interface is sampled, demodulated and delayed at the receiver, and then combined with the correspondingly processed information of the other paths to improve the quality of the received signals.
In CDMA systems, spreading is used to translate each symbol into a sequence of bits. At the transmitter, each symbol is multiplied with a wideband spreading code. The higher the bandwidth used to transmit a signal, the lower the required signal-to-noise rate must be. When a higher bandwidth is employed, the signal is spread over a greater spectrum, and that makes the signal more tolerant to interference. The spreading code is also known as the channelization code and is used to distinguish the different channels. The spreading code comprises N bits, or chips, per symbol. Each symbol to be transmitted is converted into a longer sequence of chips, by means of the multiplication. The length of the spreading code, i.e. the number of chips comprised in each spreading code, is known as the spreading factor and is dependent on the channel type. At the receiver, the original symbols can be reconstructed by multiplying the received sequence of chips with a corresponding despreading chip sequence. When the chip sequence has been demodulated at the receiver, and the corresponding symbols thus have been reconstructed, the symbols of each path are delayed with respect to the last path received, and this will compensate for the delay of each path. Note that the receiver comprises one “finger” (branch) for each path to be processed. The paths are thus time-aligned to each other and combined to form a composite symbol that is expected to have a higher signal-to-noise ratio (SNR) than a symbol received in a single path. In order to combine the different paths meaningfully, channel parameters such as number of paths, delays and attenuation of each path must be known. A major problem in particular is that the characteristics of the paths vary with time, due to the fact that the receiver is mobile. These varying characteristics must be adjusted dynamically, and that is a highly complex process.
When combining the symbols by summing each path, another problem arises. Assume that a symbol is received from a first base station (BS1) and has to be delayed and time-aligned with a subsequently received symbol from a second base station (BS2). The delay T is represented by an integral number (I) of symbol periods and a variable delay portion (D) for adjusting the alignment of symbols. If the mobile moves towards BS1 while remaining at the same distance from BS2, the delay applied to the symbol originating from BS1 should be increased to keep the two symbols aligned due to this time drift. To align a symbol received from BS1 at time t to a symbol received from BS2 at time t+D, it is necessary to adjust (delay) the symbol of BS1 with the amount D. The symbols of BS1 may then be aligned with the symbols of BS2 such that the boundary of a symbol relating to BS1 is aligned with the boundary of a symbol relating to BS2. However, a problem may still arise: after the symbol of BS1 has been subject to the delay D, a BS1 symbol of order K+1 may be aligned with a BS2 symbol of order K, and not with a BS2 symbol of order K+1, which is required to produce a coherently composite symbol. The symbol of BS1 must then be delayed by I, i.e. one symbol period, in order for the BS1 symbol of order K to become aligned to the BS2 symbol of order K. The total delay T is thus I+D in this specific example. This is known as symbol alignment. Thereafter, symbols of each path may be coherently combined to create a composite symbol.
As the mobile moves towards BS1, the delay will increase and, consequently, the delay T must be increased by 2×I, 3×I, 4×I etc. Note that the adjustment delay D varies between zero and one symbol period I and must continuously be applied to the path of BS1. When implementing the delay adjustment in the mobile phone, delay line units are employed, one for each path. To comply with real-time requirements of the system in which the mobile is comprised, there is an upper limit to the amount of delay that each delay line unit can be allowed to insert.
This maximum delay is referred to as delay line maximum length, DLM. When the delay of two multi-path symbols exceeds this maximum delay, symbol alignment is no longer possible, and symbols relating to the path of BS1 will consequently be lost. This has the effect that there can be no combination of symbols from BS1 with symbols from BS2. Thus, the worst case scenario in this example is that only symbols relating to BS2 will be received.
The delay line unit of the path relating to BS1 will then be “reset”, which means that if the DLM is e.g. 5×I, the new value of the delay is set to 3×I and again, another attempt to align the path relating to BS1 with the path relating to BS2 will be undertaken, this time with new timing (delay) parameters. The resetting of the delay line has the effect that the symbols in delay line positions 4×I and 5×I are lost.
A more preferable way to perform the symbol alignment is to, once again, to make an adjustment of the path pertaining to BS1 by applying the delay D such that the boundary of a symbol relating to BS1 is aligned with the boundary of a symbol relating to BS2. Thereafter, as in the previous example, a BS1 symbol of order K+1 may be aligned with a BS2 symbol of order K, and not with a BS2 symbol of order K+1, which is required to produce a coherently composite symbol. The symbol of BS1 must then be delayed by I, i.e. one symbol period, in order for the BS1 symbol of order K to become aligned with the BS2 symbol of order K. However, if the maximum delay is reached, a coherent adding of symbols can no longer be accomplished. Therefore, both paths are subject to delays, and when the maximum delay is reached, the symbol of order K is omitted in both paths. This has the practical effect that the path of BS1 can be delayed one more symbol period at the expense of a completely lost symbol of order K (and of course at the expense of a slight delay of received symbols for the path pertaining to BS2 as these are no longer received in real-time). The omitting of symbols is referred to as time drift compensation.
If the mobile moves away from BS1 while remaining at the same distance from BS2, the delay applied to the symbols originating from BS1 should be decreased to keep the paths aligned due to this time drift. This case is symmetrical to the case where a symbol is omitted in both paths. Consequently, the symbol of order K relating to BS1 will not be omitted, but employed twice, i.e. summed twice with the symbol of order K pertaining to BS2.
In practice, not only two, but N different paths are coherently summed. Two major problems arise when combination of symbols is to be undertaken. First, the instant in time when symbols are to be omitted depends on path timing characteristics, and will vary for all paths. Second, in the above examples, it was assumed that BS2 was the reference while the timing of BS1 was adjusted. However, the symbols derived from BS2 may disappear due to fading, which has the effect that timing control and decisions taken to omit/repeat symbols must be done in relation to a new reference path. These problems will make the management of time-drift compensation difficult.
Implementation of a common delay line located before the demodulators and processing units has been described in French patent application No. 02 10452 of 21 Aug. 2002. The main difference compared to the previous delay line implementations is that, in this case, the delays are applied to the chips rather than to the symbols. This has a major impact on performance loss of the receiver, since a chip is omitted instead of a complete symbol, i.e. only 1/N out of a symbol consisting of N chips is omitted/repeated by this operation. The delay adjustments will occur at a factor N more occasions than in the case where the delays are applied on a symbol level, but these N compensations are performed on different symbols, so the modifying of chips is spread over N symbols, which minimizes the performance loss. Another major difference is that, at the output of the delay line, the fingers of the receiver are positioned to compensate for the propagation delays, so that the chips that originate from the respective propagation paths are output at the same instant of time. This has the effect that the position of each finger is changed simultaneously when a chip is omitted/repeated. This simplifies the software management as compared to prior art not employing a common delay line.
However, the receiver disclosed in French patent application No. 02 10452 of 21 Aug. 2002 still presents unacceptable performance losses. Another problem is that the software management, even though it has been simplified, is still rather complex.